Method for crucible-free zone melting using a displaced heater

ABSTRACT

IN A METHOD OF ZONE MELTING A SEMICONDUCTOR ROD, THE ROD IS SUBSTANTIALLY VERTICALLY SUPPORTED AT ONE END BY A FIRST END HOLDER LOCATED IN THE SUBSTANTIALLY VERTICAL AXIS OF THE ROD, A SEED CRYSTAL BEING ATTACHED TO THE OTHER END OF THE ROD AND HAVING A FREE END SUPPORTED BY A SECOND END HOLDER, AND A MOLTEN ZONE IS FORMED IN THE ROD BY AN ANNULAR HEATING DEVICE SURROUNDING THE ROD AND DIVIDING IT INTO TWO ROD PORTIONS, THE ANNULAR HEATING DEVICE AND THE ROD BEING MOVED RELATIVE TO ONT ANOTHER IN THE AXIAL DIRECTION OF THE ROD FROM THE SEED CRYSTAL TO THE FIRST END HOLDER SO THAT ONE OF THE PORTIONS OF THE ROD IS SUPPLIED TO THE MELT AND THE OTHER PORTION THEREOF CRYSTALLIZES OUT OF THE MELT. THE IMPROVEMENT IN THE METHOD INCLUDES LATERALLY DISPLACING THE ANNULAR HEATING DEVICE TO A POSITION ECCENTRIC TO THE AXIS OF THE ROD BUT NEVERTHELESS STILL SURROUNDING THE ROD, AND MAINTAINING THE ECCENTRIC RELATIONSHIP OF THE ANNULAR HEATING DEVICE TO THE ROD FOR AT LEAST PART OF THE MOLTEN ZONE PASS ALONG THE ROD.

Aug. 22, 1972 w, KELLER ETAL METHOD FOR CRUCIBLE-FREE ZONE MELTING USING A DISPLACED HEATER Original Filed June 14, 1967 United States Patent U.S. (Clo 23-301 SP 8 Claims ABSTRACT OF THE DISCLOSURE In a method of zone melting a semiconductor rod, the rod is substantially vertically supported at one end by a first end holder located in the substantially vertical axis of the rod, a seed crystal being attached to the other end of the rod and having a free end supported by a second end holder, and a molten Zone is formed in the rod by an annular heating device surrounding the rod and dividing it into two rod portions, the annular heating device and the rod being moved relative to one another in the axial direction of the rod from the seed crystal to the first end holder so that one of the portions of the rod is supplied to the melt and the other portion thereof crystallizes out of the melt. The improvement in the method includes laterally displacing the annular heating device to a position eccentric to the axis of the rod but nevertheless still surrounding the rod, and maintaining the eccentric relationship of the annular heating device to the rod for at least part of the molten zone pass along the rod.

Our invention is a method of crucible-free zone melt- This method is to be carried out by zone melting apparatus of the general type illustrated and described in US. Pat. Nos. 2,972,525, 2,992,311 and 3,030,194, for example.

When performing a crucible-free zone melting process to produce crystalline rods, especially semiconductor rods having large cross sections, such as silicon rods for example of more than 25 mm. diameter, by employing an annular heating device, such as an induction heating coil, the crystals tend at times to travel or stray laterally or transversely to the axis of the rod during the growth thereof so that, after completion of the zone melting operation, the rods have a substantially corkscrew shape. This undesired deformation is observable especially when the crystallizing rod portion is set into relatively rapid rotation or is after-heated so that the liquid-solid boundary surface constituting the solidifying front is substantially planar.

It is accordingly an object of our invention to provide method for crucible-free zone melting which avoids the aforementioned undesired deformation of the crystallizing rod.

With the foregoing and other objects in view, we provide in accordance with our invention method of crucible- 3,685,973. Patented Aug. 22, 1972 in the axial direction thereof from the seed crystal to the free end of the rod due to relative motion between the heating device and the rod so that one portion of the rod supplies the melt and the other portion thereof crystallizes out of the melt. In accordance with the invention we displace the annular heating device or heating coil laterally or transversely to the rod axis into an eccentric position with respect to the axis of the rod for at least part of the melting zone pass along the rod. The lateral displacement of the annular heating device or heating coil has natural limits dictated by the fact that the heating coil or heating device must not come into engagement either with the rod or the melting zone.

The rate of displacement of the rod or the end holders thereof and the heating device or heating coil relative to one another can be predetermined so that the diameter of the rod portion resolidifying from the melt has a predetermined value which may be greater than the inner diameter of the annular heating device. Also, the diameter of the rod portion supplied to the melt, the socalled supply rod, can be greater than the inner diameter of the heating coil so that the melting zone is strangled or necked down in the vicinity of the coil and consequently has particular stability against dripping of the melt. The rod portion recrystallizing from the melt can be selectively disposed above or below the heating device and concentric or eccentric to the rod portion supplying the melt.

In accordance with further features of our invention, if desired, the heating device can be laterally displaced with respect to the rod before the seed crystal is fused to an end of the rod; for example it can be laterally displaced up to an eccentricity of 5 mm. when employing an induction heating coil having an inner diameter of between 15 and 40 mm. This eccentricity can then be maintained during the entire zone melting process. For example, to produce a silicon rod of 44 mm. diameter from a supply rod held concentric or coaxial thereto and having a diameter of 27 mm., the heating coil, in the shape of a flat coil with an inner diameter of 31 mm. and an outer diameter of 56 mm. is laterally displaced about 3 mm. or previously installed with such an eccentricity.

Other features which are considered as characteristic for the invention are set forth in the appended claims.

Although the invention is illustrated and described herein as method of crucible-free zone melting, it is nevertheless not intended to be limited to the details shown, since various modifications may be made therein without departing from the spirit of the invention and within the scope and range of the equivalence of the claims.

The method of the invention, together with additional objects and advantages thereof, will be best understood from the following description when read in connection with the accompanying drawings, in which:

FIGS. 1 and 2 are elevational views partly broken away and partly in section showing two phases during the practice of one mode of the method of our invention, wherein the relative displacement of the heating device to the rod is in an upward direction;

FIG. 2a is a diagrammatic plan view, partly in section, showing various phases during the practice of a mode of the method of our invention wherein the heating coil is rotated;

FIGS. 3 to 5 are elevational views partly broken away and partly in section showing three phases during the practice of another mode of the method of our invention wherein the relative displacement of the heating device to the rod is in a downward direction.

Referring now to the drawings and first particularly to FIG. 1 thereof there is shown a silicon rod 2 having a diameter D to which there is fused at the lower end thereof as shown in FIGS. 1 and 2, a seed crystal 3 having a diameter D corresponding to a cross section that may be one tenth or less than that of the rod cross section. With the aid of an annular heating device or flat heating coil 4, energized by a high frequency alternating current for example, there is produced a melting zone in the rod 2 which can be passed through the rod 2 along the length thereof by maintaining stationary the holders 6 at the ends of the rod and seed crystal respectively while moving the heating coil 4 upwardly as shown in FIGS. 1 and 2 or by holding the heating coil 4 stationary and moving the holders 6 downwardly or by both moving the heating coil upwardly and the holders 6 downwardly simultaneously. The seed crystal 3 can be monocrystalline for effecting monocrystalline growth. The seed crystal 3 and the resolidified rod portion 2a therewith are rotated about their longitudinal axis. In the phase shown in FIG. 1, the melting zone is located at the transition boundary between the seed crystal 3 and a rod 2 of relatively greater thickness. The ensuing method steps are represented by the arrows in FIG. 1. The heating coil 4 is displaced not only upwardly with respect to the stationary rod 2, but also simultaneously laterally or transversely thereto, for example toward the left-hand side of FIG. 1, so that the melting zone 5 is also forced toward the left-hand side of FIG. 1 due to the magnetic forces applied thereto. In FIG. 2 there is shown a phase of one mode of the method of the invention in which a predetermined nominal diameter D of the recrystallizing rod portion 2a is attained. As soon as the nominal diameter D of the recrystallized portion 2a is reached, the heating coil 4 is no longer moved transversely to the axis of the rod but is only moved upwardly as shown in FIG. 2. If the recrystallizing rod portion 2a is to be thicker than the supply rod portion 2b, as shown in FIG. 2, the upper end holder 6 (FIG. 1) for the supply rod portion 2b is continually moved closer to the melting zone with suitable velocity in the axial direction of the rod.

For more uniform distribution of doping or recombi-- nation centers or both which form impurities over the cross section of the rod, the heating coil 4 can again be moved back during the zone-melting process to a position in which it is less eccentric to the rod or can be moved back altogether to its original position in which it is coaxial with the rod or can be moved back and forth repeatedly between this position and positions at which it has predetermined eccentricity relative to the rod or between positions in which it has predetermined maximum and minimum eccentricities relative to the rod. The speed of the back-and-forth or reciprocatory motion of the heating coil 4 is advantageously great relative to the speed at which the melting zone 5 is passed through the rod 2. For example, a coil 4 can be reciprocated transversely to the rod 2 at a rate of nine (9) alternations per minute between the maximum and minimum eccentricity of the coil 4 relative to the rod 2 at the same time that the rod is being pulled in the direction of its longitudinal axis at a rate of about two (2) millimeters per minute. If the rotational speed of the recrystallizing rod portion 2a is revolutions per minute, the phase relationship of the rod portion 2a and the coil 4, starting from the position of maximum eccentricity therebetween, is altered by the passage sequentially of the recrystallized rod portion 2a through various angular positions, and the original phase relationship of maximum eccentricity between coil 4 and rod portion 2a is attained only when the coil is making its 9th alternation as it is being laterally reciprocated.

If the reciprocatory motion of the heating coil 4, for example, is forced to carry out a substantially sinusoidal course with the air of an eccentric drive mechanism, vibration or shock to the zone-melting apparatus can be greatly avoided. The can be achieved even better as shown in the plan view of FIG. 2a, by having the heating coil 4 effect a circular movement about an axis IV lying outside the axis II of the recrystallized rod portion 2a and extending parallel thereto, at constant peripheral or tangential velocity on the circular path described by each point of the coil 4. This can be achieved for example by a parallel guide with the aid of a second eccentric having the same eccentricity and the same peripheral velocity as the drive eccentric, and having an angular position conforming therewith. The circle K represents the rotary path of the coil axis III about the axis IV.

In FIGS. 3 to 5, those members corresponding to the members shown in FIGS. 1 and 2 are provided with the same reference numerals as in FIGS. 1 and 2. In the phases shown in FIGS. 3 to 5, the arrows indicate the respective directions of movement. For example, the rod portion 2b supplied to the melt 5 has a diameter of 26 millimeters, and the crystallized rod portion 2a has a nominal diameter of 33 millimeters. The respective axes of the rod portions 2a and 2b are parallel and are displaced from one another a distance of about five (5) millimeters. At the beginning of the zone-melting operation, a heating coil 4 of the flatly wound type, having an inner diameter of about 30 millimeters and an outer diameter of about 55 millimeters, are located so that the axis thereof extends in substantially the same vertical plane in which the axes of the two rod portions 2a and 2b extend, and the eccentricity of the coil 4 relative to the two rod portions is substantially the same, namely about 2.5 millimeters respectively. In order to attain the nominal diameter of the crystallizing rod portion 2a, the heating coil 4 is gradually displaced laterally or transversely to the axis of the rod portions, as shown in FIG. 3 for example, until it has an eccentricity of seven (7) millimeters with respect to the rod portion 2a and two (2) millimeters with respect to the rod portion 2b. It is also possible to effect the fusion of the seed crystal 3 initially with both rod portions 2a and 2b and the heating coil 4 being all arranged coaxially, and thereafter produce the aforedescribed eccentricity in the period necessary for attaining the nominal diameter of the crystallizing rod portions 2a, by continual displacement of the heating coil 4 and one or both rod portions 2a and 2b. FIG. 4 represents an instant or phase in which the nominal diameter of the crystallized rod portion 2a, and, simultaneously, the outermost limit of the lateral displacement of the heating coil 4 is achieved.

From the instant depicted in FIG. 4, the heating coil 4 is reciprocated laterally to the right-hand side of the figure a distance of about four (4) millimeters. FIGS. 4 and 5 represent both reversing points of the lateral reciprocatory motion of the coil 4. In the case of a circular movement of the heating coil 4 effected between the same limiting positions, the circular path described by the center of the coil does not surround the axis of the crystal lized rod portion 2a, as shown in FIGS. 4 and 5.

With the herein-described method of our invention, semi-conductor rods are in fact produced having specific resistance determined over the rod cross section with maximum deviations or tolerances of less than 10% and with an etch-pit density of less than 50,000 per square centimeter.

By the method of our invention, not only can straight rods be produced from straight monocrystalline or polycrystalline supply rods, but rather also deformed rods can be subsequently straightened. Furthermore, with our method, a more uniform distribution of impurities forming doping or recombination centers in the semiconductor rods can be obtained than with coaxial arrangement of the semiconductor rod and the heating coil. In addition, the formation of crystal defects can be reduced or practically completely avoided, especially when the crystallized rod portion in the vicinity of the melting zone is additionally heated with a further heating device surrounding the rod portion, which can advantageously be in the form of a radiant heating ring, having a temperature which is substantially the same as the melting temperature of the material being processed.

We claim:

1. In a method of zone melting a semiconductor rod wherein the rod is substantially vertically supported at one end by a first end holder, a seed crystal is attached to the other end of the rod and has a free end supported by a second end holder, and a molten zone is formed in the rod by an induction heating coil surrounding the rod and divides the rod into two rod portions, the induction heating coil and the rod being moved relative to one another in the axial direction of the rod from the seed crystal to the first end holder so that the molten zone passes axially through the rod, one of the portions of the rod being supplied to the molten zone and the other portion thereof being crystallized out of the molten zone, the improvement therein which comprises laterally displacing the induction heating coil in a direction transverse to the axial direction of the rod to a position in which it is eccentric to the rod though still surrounding the rod, and maintaining the eccentric position of the induction heating coil relative to the rod for at least part of the molten zone pass along the rod.

2. Method according to claim 1, wherein both rod portions have longitudinal axes laterally offset from one another, and which includes displacing the induction heating coil to a position in which the axis thereof is eccentric to the axes of the rod portions and is spaced a greater distance from the axis of the crystallizing rod portion and a smaller distance from the axis of the supply rod portion than the distance between the respective axes of both rod portions.

3. Method according to claim 1, which includes fusing the seed crystal to the other end of the rod, the seed crystal having a cross section smaller than the cross section of the rod, and moving the rod end holder relative to one another as the molten zone is being passed along the rod so that the crystallizing rod portion, beginning at the end of the rod fused with the seed crystal, is steadily increased in thickness to a nominal diameter and is maintained at that nominal diameter.

4. Method according to claim 3 which includes fusing the seed crystal to the other end of the rod with the induction heating coil surrounding the rod, and laterally displacing the heating coil for a period until the nominal diameter of the rod portion crystallizing from the molten zone is attained.

5. Method according to claim 2, which includes laterally displacing the induction heating coil before fusing the seed crystal to the rod.

6. Method according to claim 3 which includes imparting a periodic movement to the induction heating coil in said transverse direction, after the nominal diameter of the rod is attained, for repeatedly displacing it between two limiting positions in which the heating coil is eccentric to the rod.

7. Method according to claim 3 which includes setting the induction heating coil in rotary motion about an axis located outside the axis of the recrystallized rod portion and parallel thereto.

8. Method according to claim 3 wherein the heating coil is moved in said transverse direction at a velocity that is relatively great as compared to the velocity of the melting zone pass along the rod.

References Cited UNITED STATES PATENTS 2,914,397 11/1959 Sterling 23-301 3,177,051 4/1965 Scholte 23301 3,265,470 8/1966 Keller 23301 3,296,036 1/ 1967 Keller 23301 3,414,388 12/1968 Keller 23-301 3,477,811 11/1969 Keller 23-301 NORMAN YUDKOFF, Primary Examiner R. T. FOSTER, Assistant Examiner U.S. Cl. X.R. 23-273 SP 

